In the Western Cape 40 wheat farmers are working with Bayer CropScience to solve the problem of herbicide resistance, predominantly in ryegrass, said Hilton Olivier of Bayer CropScience at a recent meeting in Paarl. “High plant densities, together with resistance to most Group A and Group B herbicides, make ryegrass a serious problem in the Western Cape,” Olivier said.
He explained how the Bayvolution project combines the knowledge of producers, distributors and field agents to assess different treatments and control measures. Most producers are planting wheat, instead of sowing it. This allows farmers to use Trifluralin between plant rows, helping to some extent with weed control. But the weeds still grow in the plant rows, and their seeds remain in the soil. Bayvolution has shown available herbicides achieve a 0% to 10% success rate on post-harvest ryegrass control. “We either have to farm better, or get out of wheat,” Olivier told delegates.
Prof Stephan Powles from the School of Plant Biology at the University of Western Australia was more optimistic. “In we farm wheat on land that used to be planted with ryegrass for sheep grazing,” he said. “To manage herbicide resistance on ryegrass you need to know your enemy.” He agreed with Olivier that one of the major problems with ryegrass is its growth densities. “I’ve measured up to 70 000 ryegrass plants per square metre,” he said, adding that ryegrass’s large genetic variation and its ability to cross-pollinate are what make it such a difficult weed to control.
Two types of resistance
Prof Powles told delegates ryegrass can develop two types of resistance. “Target-site-based resistance occurs when the plant’s genes mutate to prevent the active ingredient in the herbicide from killing it. Non-target-site-based resistance is any change in the plant that prevents the active ingredient from reaching it. In cricket terms, target-site resistance is when your hand, representing the plant, mutates to such an extent that it can’t catch the ball, the active ingredient in the herbicide. Non-target-site resistance is a cricket bat, with which the ryegrass can hit the ball before it reaches it.”
Due to crosspollination, plants with either of these types of resistance normally survive, and their progeny will then have both types of resistance. Ryegrass’s adaptability has led to a situation where most herbicides that don’t kill wheat probably won’t kill ryegrass. No-till farming practices are very dependant on herbicides for weed control, compounding the problem. R yegrass can be managed with a diversified herbicide programme, said. Pre-seeding applications of glyphosate or paraquat should be rotated.
“Never use the same herbicide consecutively and never cut the application rates,” he cautioned. re-emergent herbicides such as Trifluralin or Boxer should also be rotated. “new product, KI485, will be available in Australia from 2011, so we’ll incorporate it into our rotation programme,” Prof Powles said. “For post-emergence, any product that’s still effective should be used. “When planning your crop rotation, remember any vigorous crop will manage weed populations, and barley is the most competitive. Any green plant in summer is a weed and should be sprayed with an appropriate herbicide.”
Advice from down under
Local producers spray ryegrass at the five-leaf stage, but Prof Powles recommended spraying at the two-leaf stage. esponding to questions, he admitted Australian wheat producers spray herbicides at concentrations above the legal limits in South Africa. They also plant with narrow row spacing, generally 18cm, to increase weed competition. However, as Africa’s yield per hectare is double Australia’s, such narrow spacing would cause a stubble problem here. yegrass seed doesn’t have a long lifespan in the soil and doesn’t shatter, so it’s present during wheat harvesting.
Australian producers collect 85% of ryegrass seeds using a chaff car with their harvesters. The chaff is then dumped and burned, though some producers prefer to funnel the chaff into narrow strips that are burned after summer. Baling the chaff removes up to 98% of ryegrass seeds, and generates income as animal feed. owever, South African producers said chaff cars are unavailable from equipment importers, and that the slopes on Western Cape farms might tip them over. E-mail Prof Stephan Powles at [email protected]. |fw
